To meet the demand of the data storage industry, various types of media are developed for data storage purposes, which include magnetic storage devices such as hard discs and floppy discs; optical storage discs such as CD-ROMs, CD-Rs, CD-RWs; as well as solid state memory devices, such as RAMs, ROMs, flash memories, dynamic random access memories (DRAMs).
Developed in the 1970s, the chalcogenide-based random access memory (C-RAM) is one type of the solid state memory which is electrically writeable and erasable. A C-RAM is an inexpensive, non-volatile memory device that is virtually impervious to radiation damage or hostile electronic interrogation, and therefore has commercial potential as a memory product for computing and data storage field, with applications ranging from smart appliances and desktop computers to new kinds of consumer products.
A conventional type of C-RAM device is disclosed in, for example, U.S. Pat. No. 5,912,839 issued to Ovshinsky et al, as shown in FIG. 1. This type of C-RAM device comprises a single crystal silicon semiconductor wafer 10 as a substrate, a memory material 36 of a single phase-change layer structure formed on the substrate, a first spacedly disposed contact 6 adjoining the volume of memory material 36, and a second spacedly disposed contact 8A adjoining the volume of memory material 36.
One of the key features of the C-RAM for storing data is its phase-change ability between the amorphous state and the crystalline state when an external energy is applied, such as an electrical current. In the data-writing process, a tiny volume of phase-change medium of the C-RAM memory element is melted by being heated with sufficient electrical energy to a temperature above its melting point, and rapidly cooled to room temperature to form the general amorphous state. In the data-erasing process, the phase-change medium is annealed at a temperature between the crystallizing temperature and the melting point to form the crystalline state.
One of the important criteria to access the phase-change ability is the data-transfer rate which is dependent on the attainable crystallization speed from the amorphous state to the crystalline state. Studies have shown that for the type of C-RAM device disclosed in the Ovshinsky's patent, in order to complete the phase-change process, the electrical current must have a pulse width of at least 50 to 200 nanoseconds (ns). When the pulse width is shortened beyond this limit, e.g. 20 ns, the device will not response with necessary phase change. Therefore, problems may arise, in particular, when the crystallization speed is not high enough to match the speed with which the electrical current passes over the medium. In such a case, the amorphous regions from the previous recording cannot be completely recrystallized during the data writing process. This causes data-recording distortion and a high level of noise.
One approach to increase the data transfer rate, or switching speed in C-RAM device is the adoption of stoichiometic compound phase-change materials as the memory material. Materials based on various different proportions of Ge—Sb—Te have also been investigated. Studies show that however, the switching speed of a regular stoichiometic phase-change material, such as Ge1Sb2Te4 or Ge2Sb2Te5 is about 50 ns to 100 ns. Therefore, these attempts have not presented a significant improvement on the switching speed of C-RAM devices.
There is therefore a need to provide an improved electrically writeable and erasable memory medium having a high data transfer rate, i.e. the medium is capable of changing phase in a shorter time of, for example, below 20 ns.